The present invention relates generally to modulating array transmitters, and more particularly, to an autocalibrating modulating array QAM transmitter system and method.
Heretofore, the assignee of the present invention has developed modulating array transmitters. For example, U.S. Pat. No. 5,612,651, assigned to the assignee of the present invention, discloses a xe2x80x9cModulating Array QAM Transmitterxe2x80x9d, and U.S. Pat. No. 5,381,449 issued to Jasper et al. disclose xe2x80x9cPeak to Average Power Ratio Reduction Methodology for QAM Communications Systemsxe2x80x9d, for example. However to date, there has not been any system or method that provides for autocalibration of such modulating array transmitters.
It is therefore an objective of the present invention to provide for autocalibrating modulating array QAM transmitter systems and methods.
To meet the above and other objectives, the present invention comprises autocalibrating modulating array QAM transmitter systems and methods. The present invention provides for self alignment of elements of a modulating array transmitter to maintain high waveform precision. The present invention is self correcting, requiring no manual alignment or realignment of transmitter elements, which is particularly important in satellite transmitters, for example. The present invention also maintains high precision of the communications waveform over the lifetime of the transmitter so that overall communications efficiency is maintained.
An exemplary autocalibrating modulating array transmitter system comprises a plurality of discrete, parallel quadrature power elements that each include a QPSK modulator coupled by way of a gain/phase adjustment circuit to a high-power amplifier driven to saturation. Data bits are input to a bit-to-symbol mapper whose outputs are coupled to the respective quadrature power elements. Outputs of the quadraphase power elements are combined in a passive network to produce a multilevel, high-power RF output signal. The output signal is sampled by a coupler and input to an autocalibration controller having outputs coupled to the bit-to-symbol mapper and to one of the gain/phase adjustment circuits. The autocalibration controller comprises a signle analog-to-digital converter that is used to sample the RF output signal.
An exemplary method or algorithm that is implemented in the autocalibrating modulating array transmitter system, and in particular in the autocalibration controller, comprises the following steps. A pair of quadraphase power elements is selected, one of which is adjusted, the other of which is held constant. A random decision is made to dither either the gain or the phase of the selected quadraphase power element. If the previous gain (phase) dither increases the error magnitude, the sign of the dither is reversed. The gain (phase) of the selected quadraphase power element is adjusted by the small dither amount (positive or negative).
A calibration symbol is sent, wherein the two phase-opposed output signals of the quadraphase power elements are designed to cancel each other. An analog-to-digital converter measures the error magnitude. Then, a predetermined number (105) of data symbols are sent, and the processing steps are repeated starting at the decision step, until the error magnitude is reduced below a threshold value. A new quadraphase power element is selected, and the above steps are then repeated, using the previously selected quadraphase power element as a reference, starting from the second step.